In U.S. Pat. No. 6,372,847, Wouters discloses thermoplastic olefin elastomers blends (TPO's). As explained by Wouters, “TPO's are multiphase polymer systems where the polypropylene forms a continuous matrix and the elastomers and filler are the dispersed phase.” In other words, TPO's comprise a majority amount of polypropylene and a minority amount of elastomers so as to form the desired structure. Wouters' TPO's are blends of a propylene-based polymer and an ethylene/α-olefin elastomer having a MLRA/ML ratio of at least 8 and an ethylene content of from about 74 to about 95 mole percent. MLRA/ML is a measure of polymer relaxation that Wouters uses to indicate the amount of long chain branching. Any polymer having a MLRA/MV value of less than three is considered to have an essentially linear structure (column 7 lines 21-22). The patentee asserts that the elastomers with an MLRA/ML ratio of at least 8 are required to provide adequately high levels of long chain branching. The TPO's comprising such elastomers are disclosed as having improved low temperature toughness. Additionally, the long chain branching in the Examples of Wouters was accomplished exclusively by the use of H-type branching agents such as vinylnorbornene, 5-ethylidene-2-norbornene, and norbornadiene and a vanadium catalyst, whereas the long chain branching in the Comparative Examples of Wouters was accomplished by T-type branching.
In WO 00/26268, Cady et al. disclose ethylene/α-olefin interpolymers characterized by a PRR of at least 4, an indication that long chain branching is present. An additional aspect of the disclosure is a polymer blend composition comprising said interpolymer and an amount of a crystalline polyolefin resin. The interpolymer is desirably present in an amount of less than 50 parts by weight and the crystalline polyolefin resin is desirably present in an amount of more than 50 parts by weight.
Manufacturers of elastomeric parts continue searching for elastomers with processing characteristics that allow them to attain any or all of higher rates of productivity, improved quality and broader markets, especially for compositions comprising a majority amount of an elastomer. Conventional processes used to make parts with an elastomeric composition component include, without limitation, profile extrusion, film extrusion, sheet extrusion, calendering, blow molding, blown film, and thermoforming processes. There are multiple methods for measuring whether or not a particular polymer or polymer blend will be useful for a particular process and/or part. Some examples of these measuring techniques include melt strength (MS), shear thinning index, zero shear viscosity, molecular weight, molecular weight distribution, creep resistance (hot creep and creep set), degree of long chain branching (LCB), gel content, elongation, and tensile strength. Depending on the particular application, some of these properties are more critical than others. Improvements in some of these properties have a direct affect upon productivity, quality and market breadth relative to such elastomeric parts.
When using a profile extrusion process, a manufacturer usually desires an elastomer that “shear thins” (in other words is shear sensitive) or decreases in viscosity with applied shear forces. Because pressure drop across an extruder die and amperage required to turn an extruder screw are directly related to elastomer viscosity, a reduction in elastomer viscosity due to shear thinning necessarily leads to a lower pressure drop and a lower amperage requirement. The manufacturer can then increase extruder screw speed until reaching a limit imposed by amperage or pressure drop. The increased screw speed translates to an increase in extruder output. An increase in shear thinning also delays onset of surface melt fracture (OSMF), a phenomenon that otherwise limits extruder output. Surface melt fracture is usually considered a quality defect and manufacturers typically limit extruder output and suffer a productivity loss to reach a rate of production that substantially eliminates surface melt fracture.
When producing profile extrusions with thin walls and a complex geometry, a manufacturer looks for an elastomer with high MS and rapid solidification upon cooling in addition to good shear thinning behavior. A combination of a high MS and rapid solidification upon cooling allows a part to be extruded hot and cooled below the elastomer's solidification temperature before gravity and extrusion forces lead to shape distortion. Ultimately, for broad market acceptance, a finished part should also retain its shape despite short term exposure to an elevated temperature during processing, shipping or eventual use.
Manufacturers who prepare elastomeric extruded and blown films and calendered sheets seek the same characteristics as those who use profile extrusion. An improved or increased shear thinning rheology leads to higher production rates before OSMF with its attendant variability in film or sheet thickness. A high MS promotes bubble stability in a blown film operation and provides a wide window of operations for further processing of such films via thermoforming. A high MS also promotes roll release during calendering. Rapid solidification or solidification at a higher temperature keeps an embossed calendering profile from collapsing or being wiped out. As with injection molding, an increase in creep resistance leads to an expansion of potential markets for resulting film and sheets.
Compositions having a high melt strength and creep resistance are desired in calendering and blow molding operations. In many instances, the calender rolls are fed with a composition in the form of a molten rod. This molten composition must be able to spread across the calender rolls. Additionally, after sheet formation, the hot sheet must resist creep or sagging until it cools.
Compositions having a high melt strength and creep resistance are also preferred for thermoforming applications. In addition, tensile properties of the compositions at elevated temperatures are important for these applications. For example, one method of manufacturing instrument panel skin material is to either calender or extrude embossed sheeting. The sheeting is then vacuum thermoformed to the contour of the instrument panel. One method to determine compound thermoformability is by evaluating its elevated stress-strain behavior. Often, flexible polypropylene thermoplastic (TPO) sheets are thermoformed at temperatures below the melting point of the polypropylene phase. Although the thermoforming process is one of biaxial extension, tensile tests at the thermoforming temperatures can be used to compare thermoforming and grain retention behavior. The peaks and valleys of the embossed grain are areas of greater and lesser thickness and a look at the grain shows that the valleys are narrower and less glossy than the peak areas. When a skin is thermoformed, the thinner areas will be subject to greater stress and the greater applied stress in these areas concentrates the elongation in the thinner valley areas. These areas elongate preferentially and the attractive “narrow valley, broad peak” appearance is lost, called “grain washout”—unless the material can be designed to elongate more evenly. Strain hardening is the property by which areas of material which have already been strained become stiffer, transferring subsequent elongation into areas which are as yet unstrained. Strain hardening thus allows a thermoformed skin to exhibit more evenly distributed elongation and minimized grain washout.
Various methods have been used in an attempt to improve the performance characteristics of polymers and polymer blends for these type of applications. One method is rheology modification of TPE compositions as disclosed by Heck et al. in WO 98/32795. The compositions comprise an elastomeric EAO polymer or EAO polymer blend and a high melting polymer. The compositions desirably contain the EAO polymer or EAO polymer blend in an amount of from about 50 to about 90 wt % and the high melting polymer(s) in an amount of from about 50 to about 10 wt %, both percentages being based on composition weight. The preferred elastomeric EAO polymer, before rheology modification, is a substantially linear ethylene polymer having a polymer backbone substituted with 0.01-3 long chain branches per 1000 carbons in the backbone. The rheology modification can be induced by various means including peroxides and radiation. The compositions of Heck et al. are said to exhibit a combination of four properties which make the compositions suited for high temperature processes: shear thinning index (STI) of at least (≧) 20, melt strength (MS) ≧1.5 times that of the composition without rheology modification, solidification temperature (ST)≧10° C. greater than that of the composition without rheology modification and upper service temperature (UST) limit ≧10° C. greater than that of the composition without rheology modification. However, while these compositions are useful in high temperature applications such as is used for automotive parts and boot shafts, the rheology modification is an extra step, radiation is expensive and peroxide by-products can leave undesirable residual odors. There remains a need to find cheaper and easier methods to prepare compositions with equivalent or superior performance characteristics for various high temperature applications such as extrusion, calendering, blow molding and thermoforming.